Various devices and systems have been developed for providing remote assistance to individuals. Such systems are sometimes called mobile personal emergency response systems (MPERS). Typically, a client of such a service enrolls in the service and provides emergency contact information to the provider of the service. The client carries a mobile device such as a specially-programmed cellular phone or another kind of device for contacting the service provider. When the client needs assistance, he or she uses the device to contact the service provider, who can answer questions, provide assistance such as navigational assistance, or summon help for the client if needed. For example, the service provider may contact one of the client's emergency contacts, or may summon official emergency response personnel.
As part of the service, the device may periodically determine its location. The location can be reported to the service provider, for example immediately upon the client calling the service provider. The service provider may use this location information to assist the client, for example to provide navigation assistance, or to direct additional help to the client. In some cases, the device may report its location periodically to the service provider.
Some devices may determine their location using a global positioning system (GPS) receiver. However, a GPS receiver requires a relatively large amount of power, and repeated GPS position fixes may deplete the battery of a portable device more quickly than desired.
There is a need for more energy efficient methods of estimating client location and movement.